Method of cleaning scale



Dec. 6, 1949 A. E. TYDEN 2,490,759

METHOD OF CLEANING SCALE Filed Oct. 6, 1943 If 4 in;

. i 36 i= 3 I 34 .l v as i 18 INVENTOR.

Patented Dec. 6, 1949 UNITED STATES PATENT- orrics I METHOD 0323mmscant: I

Arnold Eugn by mesne Application October s, 194:, Serial No.

In Sweden June 13, 1942 assignor,

to Bosenblad Corporaa corporation of New 3 Claims. (Cl. 159-23) Thepresent invention relates to a method of maintaining surface heatexchangers substantially clean as regards incrustations when used forbringing ascale forming liquid in indirect heat exchange relation withanother fluid for the purpose of heating, or evaporating such liquid,for instance, and particularly in the evapo ration of so called sulphitewaste liquor from the cooking stage of manufacturing sulphite celluloseby the calciumbisulphite process.

Generally it is preferred to use chemical means for removing anyincrustations formed and there have been proposed various means ofeffecting this in diiferent cases by interrupting the operation of theheat exchanger for a time and washing the heat exchange surface with asuitable chemical solution during such periods of inactivity. A drawbackcommon to the known chemical methods is, however, the loss of time whenthe apparatus is inactive since this causes a decrease in the usefulcapacity of the apparatus which may be considerable.

In other cases satisfactory cleaning may be effected simply by washingwith hot water or treatment with condensing steam and the heat exchangeprocess in question consists in heating a scale forming liquid by meansof relatively pure hot water or steam which causes practically noincrustations. It has also been proposed to effect the cleaning with thehot water or steam used as the heating medium so that this medium andthe scale forming liquid are passed alternately on opposite sides of theheat exchange surfaces so that the incrustations caused by said liquidon either side during one period are dissolved and removed by theheating medium during a following period withoutthe necessity ofinterrupting the operation. This method, which is described in U. S.Patent No. 1,006,197, means obviously a great progress where it can beused with success but unfortunately in many cases the incrustations growtoo rapidly or cannot be dissolved easily enough by such simple means toadmit perfect cleaning in the manner mentioned.

It is an object of the present invention to provide a method of cleaningwhich combines the advantages of the known methods described and can beadapted to give good results independently of the composition of theincrustations while rendering interruption of the heat exchange processunnecessary.

A further objector the-invention is to realize this by using a suitablechemically active fluid,

as heating medium or adding chemicals to the same and performing thecleaning with such v similar method to the heat treatment of sulphitewaste liquor, in a mode modified to suit that special case Furtherobjects and advantages of the invention will be apparent from thefollowing description taken in conjunction with the accompanyingdrawings, wherein:

The single figure of the drawing is a view partially in sectiondiagrammatically illustrating the application of this invention to aconventional heat exchanger.

Thus, in accordance with the invention, for heating a scale formingliquid, there is prepared a hot fluid which contains suitable chemicalscapable of removing the expected incrustations with relative ease, andsuch fluid and the liquid to be heated are caused to change paths witheach other at intervals in the heat exchange apparatus so that theheating fluid is utilized for cleaning purposes, similarly as known.

It is obvious that the method according to the invention must be, moreeifective than the known methods referred to and above all the newmethod has the advantage of being very easily modifled so as to meet thedifferent requirements of each special case.

In cases in which it is possible to use rather small quantities ofrelatively inexpensive and not very aggressive chemicals in the heatingfluid, there may continually be used a heating medium with a constantcontent of the desired chemicals. If, however, there are required veryexpensive or aggressive chemicals to obtain a suflicient cleaningeffect, a considerable saving of such chemicals may be effected if suchchemicals are supplied during relatively short periods, as needed toefiect cleaning, and another cheaper chemical is used at all othertimes, or no chemical at all is supplied to the heating medium duringsuch other times.

Another way of saving chemicalsis to circulate the crust dissolvingheating medium and as it leaves the heat exchanger in a cooled conditionheat it again by suitable means and pass it back to the heat exchangerpossibly together with a fresh supply of the same medium or any of itsconstituents, to serve repeatedly as heating medium.

According to the invention the heating medium proper may be provided inthe form of a liquid or in the form of vapours and in each casenaturally the chemicals chosen must be soluble a 3 in such liquid or inthe'condensate of the vapours, respectively.

Often it may be advantageous to use more or less impure waste vapoursfrom one or another process as heating medium and in case the heatingmedium proper has the form of vapours of any kind the chemicals to beadded may consist of a gaseous matter which is mixed with the vapoursand soluble in the'condensate thereof, or it may consist of a liquid orsolution which is either sprayed intothe vapours or distributed as i afilm over the surfaces conducting the vapours.

present or by other special chemicals during cer- Now, it is known inthe art to use waste vapours containing SOz-gas as liberated fromsubstantially degassed sulphite waste liquor when subjected toevaporation, a's-a medium for transmitting heat to such waste liquor ina further evaporation unit, for instance. However, the vapours generatedunder these. circumstances are but weakly acid and have no greatcleaning effect and the simple utilization thereof remains, outside thescope of the present invention which moreover calls for the addition ofextra, preferably concentrated, SOz-gas from another source if suchvapours are supplied as heating medium, in order to establish acontrolled suitable acidity as required to attain the desired increasedcleaning effect.

By way of illustrating a practical application of this invention thereis shown in the drawing a heat exchanger 10 having an outer shell Hwithin which is positioned a plurality of tubes l2 secured in tubesheets l3 fixed to the shell H in spaced relation to the ends thereof.Tubes l2 communicate at one end with a space I4 and at the opposite endwith a space I5,the spaces being closed by end walls 16 and I1respectively.

An inlet conduit I8 for liquid to be treated in the heat exchanger Itconnects to a two-way valve l9 which in turnis connected by a conduit 20with the space I5 and by a conduit 2| with the space 22 surrounding thetubes l2. An outlet conduit 23 for the treated liquid connects through atwo-way valve 24' with a conduit 25 communicating withthe space M and aconduit 26 communicating with the space 22.

A vaporous heating medium such as steam may be supplied to the heatexchanger ll through a. conduit 2'! controlled by a valve 28, thisconduit being connected to a two-way valve 29 communicating with space Hthrough conduit 30 4 a ing medium may be discharged from the heatexchanger ll through a conduit 32 connected to the space 22 or conduit33 connected to space ii, there being a two-way valve 34 provided forcontrolling the outlet through these two conduits. Valve 34 is connectedby a conduit 35 to two-way valve 36 by which the exhaust heating mediummay be directed through an exhaust line 31 or to the inlet 38 of acirculating pump 39 having an outlet 40. Outlet 40 connects with a heatexchanger 4| provided with an inlet 42 and an outlet 43 for conducting aheating medium therethrough, the heat exchanger 4| being also connectedthrough conduit 44 with the supply conduit 21.

Since the heat exchanger I0 is utilized in the treatment of, a liquidhaving scale forming properties and since this invention is concernedwith the removal of scale deposited on the internal surfaces of the heatexchanger, there is provided a pipe 45 controlled by a valve 45connected to supply conduit 21 and through which a scale dissolvingsolvent may be introduced and mixed with the heating medium flowingthrough supply conduit 21. The solvent may. of course be supplied topipe 45 from any desired source, such as a tank, pump or the like.

In order to permit proper drainage of the heat exchanger in when desiredthere is provided a drain pipe 41 connected to the bottom thereof andcommunicating with the space l5, this drain pipe being controlled by avalve 48 which would normally be closed but which may be opened fordraining the contents of the heat exchanger.

In operation the valves l9 and 24 may be set as shown in the drawing, inwhich case liquid to be treated will flow through inlet conduit l8 andconduit 20 into space I5 thence upwardly through tubes l2 to space i4and be discharged from the heat exchanger through conduit 25; valve 24and outlet conduit 23. During this period scale will be deposited on theinner surfaces of tubes l2 and must be periodically removed therefrom inorder to maintain the eiflciency of the heat exchanger at a relativelyhigh level. A heating medium such as steam is supplied through conduit21 and with the valves 29, 34 and 36 in the positions shown will supplysteam through conduit 3| to the space 22 surrounding the tubes l2, thusheating these tubes and the liquid flowing therethrough. The heatingmedium in the form of vapor and condensate or condensate alone ifcomplete condensation takes place, will be discharged from the space 22through conduit 32, valve 34, conduit 35, valve 35 and conduit 31. Theabove described manner of operation will continue until the scaledeposited on the inner surfaces of tubes I2 is suflicient to lower theefliciency of the heat exchanger a predetermined amount at which timethe positions of valves I9, 24, 29 and 34 will be reversed to permitflow of liquid from conduit l8 through conduit 2| into the space 22 andfrom space 22 through conduit 26, valve 24 and discharge conduit23.Likewise, the heating medium from supply conduit 21 will pass throughconduit 30 into space 14, through tubes I 2, space [5 and be dischargedtherefrom through conduit 33, valve 34, conduit 35, valve 36 and conduit31. During this cycle of operation, a liquid scale solvent is introducedthrough conduit 45 into supply conduit 21 and mixed with the heatingmedium, thus supplying to space M a mixture of vapor and liquid whichwill flow through the tubes l2 anddissolve scale deposited on the wallsthereof.

and with space 22 through conduit 3|. The heat- 74 This cycle ofoperation is continued until the scale deposited in the tubes I2 isremoved at which time of course scale will have been deposited on theexternal surface of the tubes due to circulation of the liquid to betreated through space 22. At this time the valves I9, 24, 29 and 34 areagain reversed to return to the original cycle of operation, duringwhich time scale will be removed from exterior surface of tubes l2 andthe liquid to be treated will pass through the interior of the tubes toagain deposit scale thereon.

While the principles of this invention may be carried out by dischargingthe mixture of heating medium and solvent from the conduit 31, it isalso contemplated that this mixture may be reheated and recirculatedthrough the heat exchanger in order to utilize the solvent properties ofthe mixture without the necessity of continually adding additionalsolvent. For this purpose the valve 36 may be positioned to conduct thedischarged heating medium and solvent through conduit 38 to circulatingpump 39, the discharge 40 of this pump directing the mixture throughheat exchanger 4| which raises the temperature thereof a predeterminedamount whereupon the mixture flows through conduit 44 into supplyconduit 21 and recirculates through the heat exchanger III. Thisreheating and recirculation may be carried out for each cycle ofoperation of the heat exchanger I or in other words may be utilizedregardless of which path of flow of the heating medium through the heatexchanger is utilized.

It will thus be seen that by this invention there has been provided arelatively simple system of which one practical application thereof isillustrated in the drawing and in which scale may be removed from thesurfaces of a heat exchanger in order to maintain the emcienc'y thereofat a relatively high level and in which this scale removing.

function may be accomplished without the neces sity of shutting down thesystem but while maintaining full capacity operation thereof. Likewise,means may also be provided to reheat and recirculate the heating mediumwhich incorporates a scale solvent thus precluding the necessity ofcontinually adding solvent and contributing to the economical operationof the system.

It will be obvious to those skilled in the art that various changes maybe made in the invention without departing from the spirit and scopethereof and therefore the invention is not limited by that which isshown in the drawings and described in the specification but only asindicated in the appended claims.

Thus, what I claim and desire to secure by Letters Patent is: v

1 The method of treating a liquid tending to deposit scale in a systemincluding a heat exchanger providing two diflerent passages for flow offluids in heat exchange relation comprising the steps of passing saidliquid through one of said passages, providing a vaporous heatingmedium, mixing with said medium a chemically active liquid scale solventto provide a mixture including liquid and vapor in sumcient quantity toinsure flow of the chemically active mixture in liquid 6 face of theother of said passages when passed therethrough whereby both the vaporand liquid solvent act to dissolve the scale and periodicallyalternating the paths of flow of the liquid to be treated and thechemically active heating medium through said passages to remove duringany one cycle the scale deposited in one of said passages bythe liquidflowing therethrough during the preceding cycle.

2. The method of treating a liquid tending to deposit scale in a systemincluding a heat exchanger providing two ditt'erent passages for flow offluids in heat exchange relation comprising the steps of passing saidliquid through one of said passages, providing a vaporous heatingmedium,

mixing with said medium a chemically active liquid scale solvent toprovide a mixture including liquid and vapor in suiiicient quantity toinsure flow of the chemically active mixture in liquid film form oversubstantially the entire wall surface of the other of said passages whenpassed therethrough whereby both the vapor and liquid solvent act todissolve the scale, reheating and recirculating said mixture through theexchanger, and periodically alternating the paths of flow of the liquidto be treated and the chemically active heating medium through saidpassages to remove during any one cycle the scale deposited in one ofsaid passages by the liquid flowing therethrough during the precedingcycle. I

3. The method of treating a liquid tending to deposit scale in a systemincluding a heat exchanger providing two difierent passages for flow offluids in heat exchange relation comprising the steps of passing saidliquid through one of said passages, providing a vaporous heatingmedium, forming a chemically active mixture of liquid and vapor bymixing with said medium a chemically active scale solvent soluble insaid medium to thereby form said mixture prior to the introduction ofthe mixture into said exchanger, and introducing the mixture into theexchanger in sufficient quantity to insure flow of the chemically activemixture in liquid film form over substantially the entire wall surfaceof the other of said passages when passed therethrough whereby both thevapor and liquid solvent act to dissolve the scale and periodicallyalternating the paths of flow of the liquid to be treated and thechemically active heating medium through said passages to remove duringany one cycle the scale deposited in one of said passages by the liquidflowing I therethrough during the preceding cycle.

ARNOLD EUGEN TYDEN.

REFERENCES CITED The following references are of record in the flle ofthis patent:

UNITED STATES PATENTS fllm form over substantially the entire wall sur-OTHER REFERENCES Handbook of Chemistry, Lange, fifth edition.

